The basic requirements of an acceptable prosthetic foot are that it will provide a stable support for the amputee throughout a reasonable range of activities and permit the amputee to walk with a normal stride. To achieve this normal stride, the prosthetic foot must flex during walking as the foot continually moves through the heel-strike, foot-flat and toe-off cycle. It must also throughout this cycle, provide transverse stability particularly at the toe-off, when the entire weight of the amputee is applied to the forward portion of the prosthetic foot. Prior art prosthetic feet typically are substantially transversely inflexible, which interferes with side to side balancing when walking on uneven surfaces. Unlike natural foot, the prosthetic foot does not sense or correct itself with this unevenness and an unanticipated sideways tilting of the foot at toe-off results in an imbalance at a critical portion of the stride.
Amputees are no longer satisfied to sit in a wheel chair or be content with a stilted walking motion. An amputee often strives to duplicate physical activities which were conducted before the amputation. These activities may include rigorous physical activities such as running, playing basketball and dancing.
Prosthetic feet to be commercially acceptable must duplicate the motions of the natural foot as much as possible. These motions include side to side stability at the toe section of the foot where weight can be exerted on each side of the foot. The ankle joint must have torsional flexibility transverse to the up and down motion of the ankle which pivotably lowers and raises the foot.
The added torsional motion of the joint in the artificial foot adds a degree of stress on the resilient pads between the members of the artificial foot not otherwise present in a foot that has limited motion in only the up and down direction relative to pivotable raising and lowering of the foot.
The torsional flexing must be limited and must be direction sensitive to provide a more lifelike ankle action. Secondly the resilient pad between the members of the feet must be positively anchored to resist being displaced during torsional motion of the ankle joint.
It is further required to have a commercially viable foot that has a formed symmetrical toe member that is used for both left and right feet so that inventory is reduced. The toe member must be shearable to conform to the contour of smaller sized left or right feet.